Simon Willison's Weblog: service-workers

Running Python ASGI apps in the browser via Pyodide + a service worker

2026-05-30T21:02:16+00:00

Research: Running Python ASGI apps in the browser via Pyodide + a service worker

Datasette Lite is my version of Datasette that runs entirely in the browser using Pyodide in WebAssembly.

When I first built it four years ago I used Web Workers and code that intercepts navigation operations and fetches the generated HTML by running the Python app.

This worked, but had the disadvantage that any JavaScript in <script> tags would not be executed - breaking some Datasette functionality and a whole lot of Datasette plugins.

This morning I set Claude Opus 4.8 the task (in Claude Code for web) of figuring out how to run Python ASGI apps in Pyodide using Service Workers instead, and it seems to work! Here's a basic ASGI FastCGI demo and here's a demo that runs Datasette 1.0a31.

I'm still getting my head around exactly how it works, but once I've done that I plan to upgrade Datasette Lite itself.

Tags: javascript, python, datasette, asgi, service-workers, pyodide, datasette-lite, claude-code

tiny-world-map

2024-04-21T22:11:14+00:00

tiny-world-map

I love this project. It’s a JavaScript file (694K uncompressed, 283KB compressed) which can be used with the Leaflet mapping library and provides a SVG base map of the world with country borders and labels for every world city with a population more than 48,000—10,000 cities total.

This means you can bundle an offline map of the world as part of any application that doesn’t need a higher level of detail. A lot of smaller island nations are missing entirely though, so this may not be right for every project.

It even includes a service worker to help implement offline mapping support, plus several variants of the map with less cities that are even smaller.

Via Hacker News

Tags: geospatial, javascript, mapping, svg, service-workers, leaflet

servefolder.dev

2021-12-12T18:32:38+00:00

servefolder.dev

Absurdly clever application of service workers and the file system API: you can select a folder from your computer and the contents of that folder will be served (just to you) from a path on this website—all without uploading any content. The code is on GitHub and offers a useful, succinct introduction to how to use those APIs.

Via AshleyScirra/servefolder.dev

Tags: browsers, javascript, service-workers

Introducing Partytown 🎉: Run Third-Party Scripts From a Web Worker

2021-09-23T18:29:14+00:00

Introducing Partytown 🎉: Run Third-Party Scripts From a Web Worker

This is just spectacularly clever. Partytown is a 6KB JavaScript library that helps you move gnarly poorly performing third-party scripts out of your main page and into a web worker, so they won’t destroy your page performance. The really clever bit is in how it provides sandboxed access to the page DOM: it uses a devious trick where a proxy object provides getters and setters which then make blocking API calls to a separate service worker, using the mostly-forgotten xhr.open(..., false) parameter that turns off the async default for an XMLHttpRequest call.

Via @Rich_Harris

Tags: async, javascript, webworkers, service-workers

Everyone can now run JavaScript on Cloudflare with Workers

2018-03-13T16:36:53+00:00

Everyone can now run JavaScript on Cloudflare with Workers

This is such a brilliant piece of software design: Cloudflare took the service workers spec and used it as the basis for their edge-executed JacaScript feature. This means you can run server-side JavaScript in hundreds of edge locations worldwide, applying custom dynamic logic (including additional async cached fetch() calls) with only around 1ms if additional overhead. The pricing model is a steal: $0.50 per million requests with a $5/month minimum.

Tags: cdn, javascript, cloudflare, service-workers